Preparation of hard carbon/carbon nitride nanocomposites by chemical vapor deposition to reveal the impact of open and closed porosity on sodium storage

The sodium-ion battery is a promising successor for the lithium-ion battery. Its energy density is limited by the anode, where sodium ideally is stored at low potentials vs. Na/Na+. The understanding of the fundamental relationships between material properties and sodium storage is often lagging behind materials development. There is a discord regarding the involvement of so-called "closed pores" in carbons in sodium storage. To investigate their influence, a chemical vapor deposition (CVD) process to deposit polymeric carbon nitride (p-C3N4) on hard carbon fibres of both, open and closed microporosity, is developed. High storage capacity at a low potential is only possible, when suitable, sealed pores are present. In fibers without notable gas-accessible surface, p-C3N4 is deposited on the external area, whereas in open-microporous samples the p-C3N4 phase grows in micropores. Consequently, except for the untreated fibres with closed pores, the composite with a pore gradient along the fibers is the only one in the study that is able to accommodate sodium at low potentials. Neither the remaining graphitic domains, nor the introduced p-C3N4 are able to accommodate sodium in a quasimetallic state. Finally, not only the sodium storage but also the solid-electrolyte interphase (SEI) build-up is influenced by the additional p-C3N4 layer.